Injectors for an injection system of an internal combustion engine, for example for a common rail injection system, are divided into various classes after their manufacture. This is usually done on the basis of a quantity deviation between a setpoint quantity of fluid and a quantity of fluid of the injector which is actually output.
In the case of conventional classification, the injector is actuated with a test control signal. The injector outputs a quantity of fluid on the basis of the test control signal. In order to permit comparisons to be made between the individual injectors, identical conditions must be present in all cases during an injector test. For this purpose, the test control signal is, in particular, a signal with in each case the same voltage, the same current strength and the same duration.
Furthermore, the injectors are each subject to the same pressure conditions as an ideal injector which serves as a reference.
The quantity of fluid which is output by the injector can be differentiated from a setpoint quantity of fluid, associated with the test control signal, of an ideal injector or reference injector. On the basis of the quantity of fluid which is output, the injector is classified and a necessary control signal is determined. In particular, each class is assigned a correction value for a duration of the control signal. This means, for example, that an injector which outputs a smaller quantity of fluid than the setpoint quantity of fluid is actuated with a control signal which is longer than the test control signal. A precondition for this is that in each case identical environmental conditions are present, as has already been stated above. In the case of an injector which outputs a larger quantity of fluid than the setpoint quantity of fluid, the behavior is actually inverted. This injector is actuated later with a control signal which is shorter in comparison with the test control signal.
In the text which follows, in particular injectors are considered which output a quantity of fluid which is larger in comparison with a setpoint quantity of fluid. If such an injector was classified with the method described above, overcompensation may occur, as a result of which in a later operating mode in an internal combustion engine the injector outputs too little or even no fluid in comparison with a setpoint quantity of fluid. As a result, a difference in quantities occurs between the setpoint quantity of fluid and the quantity of fluid which is output. During use of the injector in an internal combustion engine this may give rise to a disadvantageous combustion behavior of the internal combustion engine.
Hitherto, the above-mentioned disadvantage has not been corrected directly. Changes in the quantity of fluid which is output by an injector have hitherto been merely corrected by running time adaptation methods such as, for example, the MFMA (Minimal Fuel Mass Adaptation) method as a function of a running time of the internal combustion engine. With these methods, an initial conspicuous injection behavior of such an injector may therefore occur.
Furthermore, in particular in the contemporary Euro5 and in the future Euro6 applications, relatively small injection quantities with tighter tolerances will be required compared to Euro4 applications. This will lead to a situation in which an ever higher number of injectors will have to be removed from production at the end testing unless there is a change in the classification method described above.